| Tmpfs is a file system which keeps all files in virtual memory. |
| |
| |
| Everything in tmpfs is temporary in the sense that no files will be |
| created on your hard drive. If you unmount a tmpfs instance, |
| everything stored therein is lost. |
| |
| tmpfs puts everything into the kernel internal caches and grows and |
| shrinks to accommodate the files it contains and is able to swap |
| unneeded pages out to swap space. It has maximum size limits which can |
| be adjusted on the fly via 'mount -o remount ...' |
| |
| If you compare it to ramfs (which was the template to create tmpfs) |
| you gain swapping and limit checking. Another similar thing is the RAM |
| disk (/dev/ram*), which simulates a fixed size hard disk in physical |
| RAM, where you have to create an ordinary filesystem on top. Ramdisks |
| cannot swap and you do not have the possibility to resize them. |
| |
| Since tmpfs lives completely in the page cache and on swap, all tmpfs |
| pages will be shown as "Shmem" in /proc/meminfo and "Shared" in |
| free(1). Notice that these counters also include shared memory |
| (shmem, see ipcs(1)). The most reliable way to get the count is |
| using df(1) and du(1). |
| |
| tmpfs has the following uses: |
| |
| 1) There is always a kernel internal mount which you will not see at |
| all. This is used for shared anonymous mappings and SYSV shared |
| memory. |
| |
| This mount does not depend on CONFIG_TMPFS. If CONFIG_TMPFS is not |
| set, the user visible part of tmpfs is not build. But the internal |
| mechanisms are always present. |
| |
| 2) glibc 2.2 and above expects tmpfs to be mounted at /dev/shm for |
| POSIX shared memory (shm_open, shm_unlink). Adding the following |
| line to /etc/fstab should take care of this: |
| |
| tmpfs /dev/shm tmpfs defaults 0 0 |
| |
| Remember to create the directory that you intend to mount tmpfs on |
| if necessary. |
| |
| This mount is _not_ needed for SYSV shared memory. The internal |
| mount is used for that. (In the 2.3 kernel versions it was |
| necessary to mount the predecessor of tmpfs (shm fs) to use SYSV |
| shared memory) |
| |
| 3) Some people (including me) find it very convenient to mount it |
| e.g. on /tmp and /var/tmp and have a big swap partition. And now |
| loop mounts of tmpfs files do work, so mkinitrd shipped by most |
| distributions should succeed with a tmpfs /tmp. |
| |
| 4) And probably a lot more I do not know about :-) |
| |
| |
| tmpfs has three mount options for sizing: |
| |
| size: The limit of allocated bytes for this tmpfs instance. The |
| default is half of your physical RAM without swap. If you |
| oversize your tmpfs instances the machine will deadlock |
| since the OOM handler will not be able to free that memory. |
| nr_blocks: The same as size, but in blocks of PAGE_SIZE. |
| nr_inodes: The maximum number of inodes for this instance. The default |
| is half of the number of your physical RAM pages, or (on a |
| machine with highmem) the number of lowmem RAM pages, |
| whichever is the lower. |
| |
| These parameters accept a suffix k, m or g for kilo, mega and giga and |
| can be changed on remount. The size parameter also accepts a suffix % |
| to limit this tmpfs instance to that percentage of your physical RAM: |
| the default, when neither size nor nr_blocks is specified, is size=50% |
| |
| If nr_blocks=0 (or size=0), blocks will not be limited in that instance; |
| if nr_inodes=0, inodes will not be limited. It is generally unwise to |
| mount with such options, since it allows any user with write access to |
| use up all the memory on the machine; but enhances the scalability of |
| that instance in a system with many cpus making intensive use of it. |
| |
| |
| tmpfs has a mount option to set the NUMA memory allocation policy for |
| all files in that instance (if CONFIG_NUMA is enabled) - which can be |
| adjusted on the fly via 'mount -o remount ...' |
| |
| mpol=default use the process allocation policy |
| (see set_mempolicy(2)) |
| mpol=prefer:Node prefers to allocate memory from the given Node |
| mpol=bind:NodeList allocates memory only from nodes in NodeList |
| mpol=interleave prefers to allocate from each node in turn |
| mpol=interleave:NodeList allocates from each node of NodeList in turn |
| mpol=local prefers to allocate memory from the local node |
| |
| NodeList format is a comma-separated list of decimal numbers and ranges, |
| a range being two hyphen-separated decimal numbers, the smallest and |
| largest node numbers in the range. For example, mpol=bind:0-3,5,7,9-15 |
| |
| A memory policy with a valid NodeList will be saved, as specified, for |
| use at file creation time. When a task allocates a file in the file |
| system, the mount option memory policy will be applied with a NodeList, |
| if any, modified by the calling task's cpuset constraints |
| [See Documentation/cgroup-v1/cpusets.txt] and any optional flags, listed |
| below. If the resulting NodeLists is the empty set, the effective memory |
| policy for the file will revert to "default" policy. |
| |
| NUMA memory allocation policies have optional flags that can be used in |
| conjunction with their modes. These optional flags can be specified |
| when tmpfs is mounted by appending them to the mode before the NodeList. |
| See Documentation/vm/numa_memory_policy.txt for a list of all available |
| memory allocation policy mode flags and their effect on memory policy. |
| |
| =static is equivalent to MPOL_F_STATIC_NODES |
| =relative is equivalent to MPOL_F_RELATIVE_NODES |
| |
| For example, mpol=bind=static:NodeList, is the equivalent of an |
| allocation policy of MPOL_BIND | MPOL_F_STATIC_NODES. |
| |
| Note that trying to mount a tmpfs with an mpol option will fail if the |
| running kernel does not support NUMA; and will fail if its nodelist |
| specifies a node which is not online. If your system relies on that |
| tmpfs being mounted, but from time to time runs a kernel built without |
| NUMA capability (perhaps a safe recovery kernel), or with fewer nodes |
| online, then it is advisable to omit the mpol option from automatic |
| mount options. It can be added later, when the tmpfs is already mounted |
| on MountPoint, by 'mount -o remount,mpol=Policy:NodeList MountPoint'. |
| |
| |
| To specify the initial root directory you can use the following mount |
| options: |
| |
| mode: The permissions as an octal number |
| uid: The user id |
| gid: The group id |
| |
| These options do not have any effect on remount. You can change these |
| parameters with chmod(1), chown(1) and chgrp(1) on a mounted filesystem. |
| |
| |
| So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs' |
| will give you tmpfs instance on /mytmpfs which can allocate 10GB |
| RAM/SWAP in 10240 inodes and it is only accessible by root. |
| |
| |
| Author: |
| Christoph Rohland <cr@sap.com>, 1.12.01 |
| Updated: |
| Hugh Dickins, 4 June 2007 |
| Updated: |
| KOSAKI Motohiro, 16 Mar 2010 |